There is increasing evidence that myosin has associated with it in a noncovalent manner small molecular weight proteins, "light chains" of about 20,000 daltons, and that the association is necessary for the biological activity (ATPase and actin binding) of the contractile protein. Recent experiments by this investigator have shown first, that myosin main ("heavy") and light chains are synthesized separately and second, that the two myosin components are being synthesized at different rates implying that they are turning over at different rates. The proposed research will utilize two systems, embryonic muscle in which the assembly of the contractile unit is being examined and adult muscle in which normal protein turnover (replacement) is being examined. The experiments will be concerned with: 1. the control mechanisms involved in the separate but presumably coordinated synthesis of the two myosin components; 2. the problem of whether the contractile unit of muscle turns over as a whole or in parts (which will involve turnover studies not only of myosin heavy and light chains but also tropomyosin, actin, the troponins and the actinins); 3. the involvement of myosin light chains in several of the abnormal or diseased states of muscle (dystrophy, hypertrophy, atrophy) and with the trophic effects of nerve on muscle. The studies for the most part will involve an analysis of the distribution of radioactive amino acids in protein after both long and short term administration of label. Myosin ATPase is probably the rate-limiting step in the contractile process: hence an understanding of the relationship between the synthesis and metabolism of its heavy and light chain components is of crucial importance to understanding the assembly of myosin in developing and adult tissues and to an understanding of the maintenance of normal muscle contractility. It is likewise of interest to learn if changes in the light chain fraction of myosin are involved in any of the diseased states of muscle or in the trophic effects of nerve on muscle. An answer to the question of whether or not the contractile unit of muscle turns over as a whole or by parts is requisite to an understanding of both normal and diseased muscle metabolism.